Sequentially connecting optocouplers for better isolation slows things way down, digital isolators don’t suffer as much. The advantages of digital isolators over opto-couplers [PDF!] in layered isolation:
Creating double insulation barriers with sequential optocouplers is problematic because the data integrity is poor, and there is no compact inexpensive way to provide power to the interface between the barriers. With the advent of high performance digital isolators like iCoupler, and integrated power in isoPower devices, creating high voltage isolation
barriers by layering isolators is now a viable solution.